Abstract The effects of varying fat content in Cheddar cheese, from 6.3 to 32.5 g 100 g −1 , on changes in pH, primary proteolysis and texture were monitored over a 225 d ripening period. Reduction in the fat content resulted in significant ( P α s1 -casein was degraded more slowly, and β -casein more rapidly, as the fat content was reduced. The slower degradation of α s1 -casein with decreased fat content coincided with a decrease in the ratio of residual chymosin activity to protein in the cheese. At most ripening times, reduction in the fat content resulted in significant increases in the concentration of intact casein, fracture stress, fracture strain, and cheese firmness. The effects of fat reduction on proteolysis and rheology are probably due to the interactive effects of the concomitant changes in composition.